Method and system for the removal of particulate matter and heavy metals from engine exhaust gas

ABSTRACT

Method and system for removal of soot, ash and heavy metals, and optionally additionally NOx and SOx being present in exhaust gas from an engine operated on heavy fuel oil.

The present invention relates to a method and system for the removal of hydrocarbons and particulate matter in form of soot, ash and heavy metals being present in exhaust gas from an engine. In particular the invention is useful for the removal of these components from the exhaust of an engine operated on heavy fuel oil.

Soot and ash are typically captured and removed by passing the exhaust through one or more filters arranged in the exhaust system. After a certain time on stream the captured amounts of soot and ash cause an increasing pressure drop over the filters and the filters need to be regenerated by burning off the soot and blowing off the ash with compressed air or by a manual process.

The known particulate filter systems are developed for diesel engine exhaust with a relatively low content of sulphur compounds and particulate matter. These systems can not be employed for e.g. maritime engines fuelled with heavy fuel oil, the so called bunker oil.

Bunker oil contains very heavy hydrocarbons and polyaromatic compounds. The oil is heavily contaminated with compounds, which do not burn and end as ash in the exhaust. Further contaminants contained in bunker oil include not only water soluble metal salts sodium (Na), potassium (K), calcium (Ca), iron (Fe), sulfates (MeSO₄), and several others, but also the oil soluble metals vanadium (V), lead (Pb), nickel (Ni) and others.

Thus, the general object of the invention is to provide a method and system for cleaning exhaust gas resulting from engines being fuelled with heavy fuel oil, which method and system ensure an effectively cleaning and a continuous operation of the engine, even when a particulate filter employed in the method and system needs to be regenerated.

As discussed above particulate matter in the exhaust gas from the engine contains further inorganic ash that cannot combust and therefore will accumulate in the filter over time and build up the pressure drop. Consequently, the inorganic ash and remaining amounts of soot must be removed by periodical reversing the flow direction of the exhaust gas through the filter or blowing off the ash and soot by impulsed injection of air.

Essential features of the invention are a continuous passive regeneration of particulate filters by catalysing the filters with soot combustion and hydrocarbon oxidation catalysts, thereby improving the fuel consumption by keeping the pressure drop over the particulate filters low and by periodically and effectively blowing off of particulate matter by pulse injection of air into outlet of the filters. The catalysts facilitate to burn off and substantially remove sticky hydrocarbon containing soot that facilitates the ash removal.

In summary, the invention provides a method for removal particulate matter, hydrocarbons, and heavy metals being present in exhaust gas from an engine operated on heavy fuel oil, comprising the steps of

operating the engine at a load to obtain an exhaust temperature of the exhaust gas of at least 325° C.;

passing the exhaust gas at exhaust gas temperatures of 325° C. to 550° C. through the at least one filter unit each comprising at least one particulate filter and capturing the particulate matter and heavy metals contained in the exhaust gas;

continuously burning the captured soot and adhered hydrocarbons off the at least one particulate filter by contact with a catalyst being arranged on the particulate filter; periodically disconnecting the at least one filter unit from flow of the exhaust gas;

applying a pneumatic pulse at the outlet of the at least one particulate filter by pulse injecting air into the outlet in reverse to the previous flow of the exhaust gas and blowing off the captured particulate matter together with the heavy metals from the at least one particulate filter;

applying suction at inlet of the at least one particulate filter;

and conveying the blown off particulate matter and heavy metals from the at least one particulate filter, optionally through an external auxiliary filter unit, to a container.

Preferred embodiments are disclosed in the following. These embodiments can either be employed each alone or in combination thereof.

The particulate filters for use in the invention are preferably made from silicon carbide, cordierite, mullite, aluminium titanate or sintered metal.

Typically, the filters for use in the invention are shaped as wall flow filters, which ensure the highest cleaning efficiency, but other filter types may be employed.

The soot combustion catalyst is coated on and/or inside the filter walls.

Catalysts being active in the combustion of soot are known in the art and described inter alia in the patent literature.

A preferred catalyst comprises titanium dioxide, oxides of vanadium and tungsten and metallic palladium as further disclosed in European patent no. EP1493484 B1.

The catalyst reduces the ignition temperature of the trapped soot down to 350° C. and at optimal process conditions further down to 325° C.

An auxiliary engine can be operated at a part load, whereby the exhaust gas temperature is above 325° C. The exhaust gas temperature above 325° C. at filter inlet thus secures passive regeneration by continuous soot combustion.

Heavy fuel oil contains large amounts of vanadium and iron acting as fuel born additives and facilitate additionally burning off the soot above 325° C. and thereby make it unnecessary to add additives to the fuel.

As mentioned above an essential feature of invention is removal of trapped particulate matter formed during combustion of the heavy fuel oil. The particulate filters must be cleaned periodically by shutting off the filters from exhaust gas flow.

All filter units can be cleaned in a cyclic cleaning loop by the method according to the invention as described in more detail in the following. The engine can remain in continuous operation because at least one filter unit remains in filtration mode.

During cleaning of the particulate filters, air is injected in reverse to the previous flow of the exhaust gas at an injection pulse duration of between 10 and 600 msec, preferably 300 msec.

In the cleaning cycle, a particulate filter or a section of a filter unit (5-50% of the entire filtering surface) can be closed at the outlet and the air is injected into the outlet by a valve or nozzle e.g. mounted on or near a closing valve. Thereby blowing off particulate matter comprising ash, uncombusted soot and heavy metals trapped in the particulate filter is even more efficiently because of the lower volume the air pulse injected into the particulate filter compared to a manner, where the outlet is open. In the latter case, the air pulse propagates throughout the whole filter unit encasing the particulate filter/s and thus limits the cleaning effect.

Alternatively, the outlet of each particulate filter can be open during the cleaning cycle using a more powerful air injection pulse. The advantage of this embodiment is simplicity and a more compact filter arrangement.

During the pneumatic injection pulse into the particulate filter to be cleaned, a cleaning air stream with high concentration of particulate matter exits from the inlet side of the particulate filter and the air stream is captured by a proper suction system. The air stream containing the blown off particulate matter is then conveyed away from the inlet of the particulate filter through a suction pipe, optionally provided with a perforated grid installed at or close to the inlet of the particulate filter. The suction pipe is connected to a suction source e.g. a suction pump, which is activated during or after the air pulse is injected into outlet of the particulate filter.

The particulate matter may be sucked in the suction pipe through an external auxiliary low temperature filter or an auxiliary high temperature filter or both, optionally catalysed with a soot combustion catalyst as described above. Thereby, particulate matter removed from the main particulate filters and contained in the cleaning air stream is separated from the stream in the auxiliary filter/s and then discharged into a storage container for future disposal.

The suction source can be made alternatively by an external suction pump provided with a small auxiliary filter. The auxiliary filter collects the particle carried in the air stream in the suction gas flow line.

Alternatively the suction stream can be also created by utilizing the pressure drop across the particulate filter/s. In this embodiment the suction pipe connects the exhaust gas inlet side of the filter unit/s or the particulate filter/s with the exhaust gas outlet side from the filter unit/s or the particulate filter/s and the particulate matter blown off the particulate filter/s is sucked through an auxiliary filter installed in the suction pipe. When the cleaning cycle is in pause captured particulate matter can be removed from the auxiliary filter.

The pressure applied in the suction pipe must be low enough to ensure an efficient transport of particulate matter in the suction pipe.

Preferably the pressure in the suction pipe is in the range of 30-300 mbar below the pressure inside the particulate filters.

In a further embodiment of the invention, the air for pulse injection is withdrawn from an accumulator tank with compressed air at a pressure of 4 to 10 bar abs, preferably 6.5 bar abs.

In yet another embodiment, the unit/s are arranged in a pressure vessel upstream an engine turbocharger. The exhaust gas may then be passed through the filter unit/s at a pressure of between 0 and 3 bar abs.

The soot combustion temperature can in this embodiment be kept at a more optimal level about 400° C. without additional exhaust gas heating. As further an advantage, pressure drop over the particulate filter(s) is decreased when increasing the pressure of the exhaust gas and the temperature. The latter result advantageously in a diminished particulate filter volume required for an effective filtration and facilitates e.g. a retrofit installation on ships with limited space for exhaust gas treatment.

The filtration process is in yet another embodiment additionally combined with selective catalytic reduction (SCR) of nitrogen oxides (NOX) in the exhaust gas prior to the gas is passed through the filter unit/s or after the gas has passed through the filter unit/s.

When the engine is operated above a minimum load resulting in an exhaust gas temperature of at least 325° C. the thermal mass of the SCR unit has a negligible effect on the passive regeneration of the downstream filter unit/s.

An important feature of the invention as disclosed above is the possibility to remove sulphur oxides being formed when burning heavy fuel oil in the engine. The upstream soot burning catalyst is resistant to sulphur compounds and has a limited SO₂ to SO₃ oxidation potential.

Thus, in a further embodiment the method comprises the additional step of reducing amounts of sulphur oxides contained in the exhaust gas by scrubbing the gas in an open or closed loop, downstream of the at least one filter unit with a scrubbing liquid comprising an aqueous alkaline solution or an alkaline solution in sea water. In the alkaline scrubbing liquid the sulphur oxides are converted to harmless alkaline metal sulphates or sulphites. The sulphur oxides are thereby almost completely removed and a clear low turbidity spent solution is stored for delivery onshore.

As a further advantage, seawater can be used to in the scrubbing liquid. As heavy metals and soot are removed by the filters, the captured sulphur content in the scrubbing liquid can then be appropriately diluted for pH control and discharged into the sea.

The invention provides furthermore a system for removal of particulate matter comprising soot, ash and heavy metals being present in exhaust gas from an engine operated on heavy fuel oil comprising

one or more exhaust gas inlet pipes connecting each the engine with inlet of each of one or more filtration units; one or more exhaust gas outlet pipes connected to outlet of each of the one or more filtration units;

arranged within the one or more filtration units at least one particulate filter catalyzed with a catalyst for effectuating burning off of soot and hydrocarbons;

an air pulse jet arrangement mounted at the outlet of the at least one particulate filter for blowing off the particulate matter collected at the at least one particulate filter;

and the air pulse jet arrangement comprises

one or more air blow pipes connected to an air supply, nozzles in the air blow pipes and an eductor arranged at outlet of the least one particulate filter for pulse injection of air into the at least one particulate filter; and

a suction pipe installed close to the inlet of the least one particulate filter, the suction pipe being connected to a suction source.

The reverse flow through the filter is controlled by automatic managing the jet valves at filter outlet as described in detail below by reference to the drawings in which FIG. 1 schematically shows operation of the system according to an embodiment of the invention.

Preferred embodiments are disclosed in the following. These embodiments can either be employed individually or in combination thereof.

The at least one particulate filter is in form of a wall flow filter.

The at least one particulate filter is coated on walls or inside walls with a catalyst catalysing burning of captured soot with adhered hydrocarbons of the filters.

The catalyst consists preferably of titanium dioxide, oxides of vanadium and tungsten and metallic palladium.

The substrate of the at least one particulate filter may be prepared from silicon carbide, cordierite, mullite, aluminium titanate or sintered metal.

The one or more air blow pipes are connected to an accumulator tank with compressed air.

The one or more filtration units are arranged in a pressure vessel upstream an engine turbocharger.

The one or more filtration units are arranged downstream an engine turbocharger.

The one or more exhaust gas outlet pipes connect the one or more filtration units to a downstream selective catalytic reduction unit comprising a denitrification (SCR) catalyst.

The one or more exhaust gas inlet pipes connect the one or more filtration units to an upstream selective catalytic reduction unit comprising a denitrification (SCR) catalyst.

The one or more exhaust gas outlet pipes connect the one or more filtration units to a scrubber unit.

A selective catalytic reduction unit comprising a denitrification (SCR) catalyst unit is connected upstream to the one or more filtration units and downstream to a scrubbing unit.

The selective catalytic reduction unit is arranged upstream an engine turbocharger.

The system comprises further a by-pass pipe by-passing the exhaust gas at least one of the one or more filtration units.

The system further comprises one or more auxiliary filter units connected to the perforated grid and/or pipe downstream the at least one particulate filter.

The air pulse jet arrangement further comprises an isolation valve at outlet of the at least one particulate filter.

The various advantages of the different embodiments of the system according to the invention are already discussed above in connection with the disclosure of the method according to the invention.

A more detailed description of the method and system is apparent from the following description of a specific embodiment with reference to the drawings in which

FIG. 1 shows a schematic flow sheet of the method and system according to the invention; and

FIG. 2 is en exploded view of the cleaning arrangement and the valve and nozzle configuration arranged at outlet of a particulate filter.

Referring now to FIG. 1, the system for use in the method according to an embodiment of the invention comprises a filtration unit 4 connected at outlet via exhaust turbine 12 of a turbocharger 10 to an SCR unit 6. SCR unit 6 is connected to SO_(x)—scrubber 8.

The filtration unit 4 is divided by a wall 14 into an exhaust gas inlet section 4 a and a filtrated exhaust gas outlet section 4 b. The unit 4 comprises three particulate filters 16 a,b,c.

The particulate filters are modular and spaced apart arranged in unit 4, which allows individual regeneration or replacing of spent filters as described below.

Outlets 18 a, b, c of the particulate filters are lockable and connected to pulse jet cleaning valves 20 a, b, c. The cleaning valves can lock the outlet of the filters sequentially or all at once after a predetermined time on stream or otherwise determined, e.g. by the pressure drop created over the filters. The jet cleaning valves may be connected to an accumulator tank with compressed air (not shown)and provide a pressurized and pulsed air stream with a duration as disclosed above in reverse flow to the previous exhaust gas flow through filters 16 a,b,c. By these means, ash and remaining amount of soot together with heavy metals accumulated in the filters are blown off to a discharge sluice 22. During regeneration of filters 4 a,b,c, exhaust gas flow to the actual filtration units, unit 4 in FIG. 1 is disrupted by means of valve 30 and the gas is by-passed to another filtration unit (not shown).

The filtration unit 4 is connected to a downstream air compressor 24 of a turbocharger 10 via the engine 2 by an exhaust gas pipe 26. The advantage of such a configuration is described hereinbefore.

When connected upstream of turbocharger air compressor 24, it is preferred to arrange the filtration unit 4 within a pressure vessel 28 in order to allow the filtration unit to better utilize the pressure drop gain with the same soot load obtained by the pressurized engine exhaust gas. The soot combustion increases with higher temperature that is always present upstream a turbocharger and may eliminate support heating.

The filtrated exhaust gas is passed from filtration unit 4 in line 32 via exhaust turbine 12 of turbocharger 10 to SCR catalyst unit 6. Prior to be introduced into unit 6, urea is injected into the gas as reductant for the SCR of nitrogen oxides. The SCR reaction and catalysts for use in the reaction are widely disclosed and known in the art and need no further description.

Finally, the SCR treated exhaust gas in pipe 34 is passed to scrubber unit 8 for the removal of SO_(x). In unit 8 the exhaust gas is scrubbed with a diluted alkaline solution, e.g. an aqueous solution of sodium hydroxide wherein the SO_(x) are converted to sodium sulphite and/or sodium sulphate dissolved in the scrubber solution. The pH value of spent scrubber solution can easily be adjusted to a value around 7 and because heavy metals, soot and ash have been removed from the exhaust gas prior to scrubbing it is possible to distribute spent scrubber solution into the environment with negligible risk thus fulfilling foreseen IMO regulations.

The thus cleaned exhaust gas is withdrawn from scrubber unit 8 and passed in pipe 36 to an exhaust stack (not shown).

FIG. 2 is an exploded view of an air pulse jet valve arrangement 20 connected to outlet 18 of the particulate filters 16, shown in FIG. 1.

The air pulse jet valve arrangement 20 according to an embodiment of the invention comprises air blow pipes 21 a and 21 b with air nozzles (not shown) at outlet of the pipes. The air blow pipes are connected through pipe 23 to a pressurized air supply from a compressed air tank (not shown). Valve arrangement 20 comprises further an isolation valve 25 at outlet 18 of a filtration unit 4. The filtration unit 4 is provided with two filters 16 a and 16 b with outlet pipes 19 a and 19 b, respectively. The outlet pipes are in form of eductors.

During filtration operation, the outlet 18 is open and filtered exhaust gas leaving filters 16 a and 16 b from outlet pipes 19 a and 19 b is withdrawn through outlet 18.

In regeneration mode as shown in FIG. 2, outlet 18 is locked by isolation valve 25 and pressurized air from pipe 23 is passed sequentially to air blow pipes 21 a and 21 b and pulse injected into eductors 19 a and 19 b, respectively. The air pulse injected into filters 16 a and 16 b in reverse to the previous exhaust gas flow causes ash and remaining amounts of soot accumulated in the filters to peel off from the filter surface and then being blown to a perforated grid 23 a and 23 b close to the outlet of filter 16 a and 16 b. The blown off particulate matter is sucked through the grids to suction pipe 22 connected to grids 23 a and 23 b. Suction pipe 22 is connected to a vacuum pump (not shown) establishing a sufficient suction pressure in the line to suck the particulate matter through an external filter 24. Captured particulate matter is removed from filter 24 and conveyed to a disposal container 26.

In FIG. 2, filter 16 a is under regeneration. An air pulse 27 is injected through air blow pipe 21 a into eductor 19 a in outlet of filter 16 a for about 300 msec. Particulate matter 28 is hereby blown off from filter 16 a and collected on grid 23 a facing the outlet of filter 16 a. During the air pulse or after the finished air pulse, suction is applied in line 22 and the collected particulate matter on grid 23 a is sucked in line 26 through auxiliary filter 27 and captured. The captured particulate matter is disposed to container 22. 

1. A method for removal particulate matter, hydrocarbons, and heavy metals being present in exhaust gas from an engine operated on heavy fuel oil, comprising the steps of operating the engine at a load to obtain an exhaust temperature of the exhaust gas of at least 325° C.; passing the exhaust gas at exhaust gas temperatures of 325° C. to 550° C. through the at least one filter unit each comprising at least one particulate filter and capturing the particulate matter and heavy metals contained in the exhaust gas; continuously burning the captured soot and adhered hydrocarbons off the at least one particulate filter by contact with a catalyst being arranged on the particulate filter; periodically disconnecting the at least one filter unit from flow of the exhaust gas; applying a pneumatic pulse at the outlet of the at least one particulate filter by pulse injecting air into the outlet in reverse to the previous flow of the exhaust gas and blowing off the captured particulate matter together with the heavy metals from the at least one particulate filter; applying suction at inlet of the at least one particulate filter; and conveying the blown off particulate matter and heavy metals from the at least one particulate filter, optionally through an external auxiliary filter unit, to a container.
 2. The method of claim 1, wherein the at least one particulate filter is in form of a wall flow filter.
 3. The method of claim 2, wherein the catalyst is coated on or inside the walls of the at least one particulate filter.
 4. The method of claim 1, wherein the catalyst comprises of titanium dioxide, oxides of vanadium and tungsten and metallic palladium.
 5. The method of claim 1, wherein body of the at least one particulate filter is prepared from silicon carbide, cordierite, mullite, aluminium titanate or sintered metal.
 6. The method of claim 1, wherein the air is pulse injected with injection pulse duration of between 10 and 600 msec, preferably 300 msec.
 7. The method of claim 1, wherein the air for pulse injection is withdrawn from an accumulator tank with compressed air at a pressure 4 to 10 bar abs, preferably 6.5 bar abs.
 8. The method of claim 1, wherein the at least one filter unit is arranged in a pressure vessel upstream an engine turbocharger.
 9. The method of claim 8, wherein the exhaust gas is passed through the at least one filter unit at a pressure of between 0 and 3 bar abs.
 10. The method according to claim 1, comprising the further step of selective catalytic reduction of nitrogen oxides in the exhaust gas prior to the gas is passed through the at least one filter unit or after the gas has passed through the at least one filter unit.
 11. The method according to claim 1, comprising the further step of reducing amounts of sulphur oxides contained in the exhaust gas by scrubbing the gas with an alkaline solution or sea water in an open or closed loop, downstream of the at least one filter unit.
 12. A system for removal of particulate matter comprising soot, ash and heavy metals being present in exhaust gas from an engine operated on heavy fuel oil comprising one or more exhaust gas inlet pipes connecting each the engine with inlet of each of one or more filtration units; one or more exhaust gas outlet pipes connected to outlet of each of the one or more filtration units; arranged within the one or more filtration units at least one particulate filter catalyzed with a catalyst for effectuating burning off of soot and hydrocarbons; an air pulse jet arrangement mounted at the outlet of the at least one particulate filter for blowing off the particulate matter collected at the at least one particulate filter; and the air pulse jet arrangement comprises one or more air blow pipes connected to an air supply, nozzles in the air blow pipes and an eductor arranged at the outlet of the at least one particulate filter for pulse injection of air into the at least one particulate filter; and a suction pipe installed close to the inlet of the least one particulate filter, the suction pipe being connected to a suction source.
 13. The system of claim 12, wherein the at least one particulate filter is in form of a wall flow filter.
 14. The system of claim 13, the at least one particulate filter is coated on walls or inside walls with a catalyst catalysing burning of captured soot of the filters.
 15. The system of claim 12, wherein the catalyst consists of titanium dioxide, oxides of vanadium and tungsten and metallic palladium.
 16. The system according to claim 12, wherein body of the at least one particulate filter is prepared from silicon carbide, cordierite, or mullite or aluminium titanate or sintered metal.
 17. The system of claim 12, wherein the one or more air blow pipes are connected to an accumulator tank with compressed air.
 18. The system of claim 12, wherein the one or more filtration units are arranged in a pressure vessel upstream an engine turbocharger.
 19. The system of claim 12, wherein the one or more filtration units are arranged downstream an engine turbocharger.
 20. The system of claim 12, wherein the one or more exhaust gas outlet pipes connect the one or more filtration units to a downstream selective catalytic reduction unit comprising a denitrification catalyst.
 21. The system of claim 12, wherein the one or more exhaust gas inlet pipes connect the one or more filtration units to an upstream selective catalytic reduction unit comprising a denitrification catalyst.
 22. The system of claim 12, wherein the one or more exhaust gas outlet pipes connect the one or more filtration units to a scrubber unit.
 23. The system of claim 12, wherein a selective catalytic reduction unit comprising a denitrification catalyst unit is connected upstream to the one or more filtration units and downstream to a scrubbing unit.
 24. The system of claim 20, wherein the selective catalytic reduction unit is arranged upstream or downstream an engine turbocharger
 25. The system of claim 12, further comprising a by-pass pipe by-passing the exhaust gas at least one of the one or more filtration units.
 26. The system of claim 12, further comprising one or more auxiliary filter units connected to the suction pipe.
 27. The system of claim 12, wherein the air pulse jet arrangement further comprises an isolation valve at outlet of the at least one particulate filter.
 28. The system of claim 12, wherein the suction pipe connects an exhaust gas inlet side of the filter unit/s or the particulate filter/s with an exhaust gas outlet side from the filter unit/s or the particulate filter/s. 